You have to collect six puzzle pieces before achieving this.
To obtain the first, jump into the gap in the maple woods as the picture below shows. Here lies a portal.
Enter it and struggle to reach the end point. The first piece awaits there.
To get the second, find the gap in the red bamboo forest near its entrance point. Enter the portal and walk through the long road, you will see this one.
The third one is in the Sword Tomb Region. You may encounter a puzzle that requires you to use mirrors and light to open a gate. At this spot, there is a hidden mirror in the ditch. Use it to reflect light and open the gate hidden in the cave.
Enter the portal, manipulate switches to integrate the picture below and then you can finally get the third piece.
The fourth piece is in the maple woods. You may go into a huge room where the heroine is able to see a hidden portrait. Get down to the bottom floor and you can enter a hidden room on the left. To collect the piece, you’d better get equipped with light which is two-checkpoint away.
The fifth is in the Milky Way Region. You may pass a spot shown in the picture below. Go left and open a gate, you will see the fifth piece.
The sixth is located in the factory region. You will pass two buildings in the picture before leaving the factory.
Manage to get into the semi-circular portal.
Find this chime on the background and use the trampoline to get the last piece.